Transparency painting gone wrong
It doesn't work because your &quot;transparent&quot; Principal BSDF is not transparent because the Alpha slot value is 1.0. It should be 0.0. But you don't need to mix 2 Principled BSDF shaders for transparency.
You just need one Principled BSDF. Then plug the mask (Image Texture node) into the Alpha slot of the Principled BSDF node. That's all :-)
Of course, you can do it as you did. But you better would take a Transparent BSDF (instead of another Principled BSDF) and mix it with a Principled BSDF. The mask goes into the Fac slot of the Mix Shader as you did. Such setups are often used for glass and plastic shaders.
Transparent shader nodes don't need a normal map. (At least I've never this.)